1. Field of the Invention
The present invention relates to a method for etching an insulating film and a method for manufacturing a semiconductor optical device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2000-323461 discloses one method for forming a fine pattern using a nano-imprint lithography (NIL) technique. In the method for forming a fine pattern using the NIL technique disclosed in this document, a mold having a predetermined pattern is pressed on resist applied on a substrate to transfer the predetermined pattern to the resist, forming a resist mask. Then, the substrate is processed using the patterned resist mask to form a fine pattern on the substrate.
Also, another method for forming a fine pattern using the NIL technique is described in M. Miller, et. al., “Fabrication of Nanometer Sized Features on Non-Flat Substrate Using a Nano-Imprint Lithography Process”, Proc. SPIE 5751,994, pp. 995-998, (2005). This method includes a step of forming a resist mask using two resin layers. Specifically, a first resin layer is formed on a substrate, and a mold is pressed on the first resin layer to transfer projections and recesses to the first resin layer. Next, a second resin layer is formed to cover the first resin layer and then etched back to expose the projections of the first resin layer. Then, the exposed portions of the first resin layer are selectively etched up to the surface of the substrate using the second resin layer as a mask. As a result, a resist mask including the two resin layers is formed.
The NIL technique is preferably used for, for example, forming a diffraction grating structure in manufacturing a DFB (Distributed Feedback) laser diode. Furthermore, the diffraction grating structure including phase shift or a chirped diffraction grating structure can be fabricated by using the NIL technique. The diffraction grating structure includes periodic projections and recesses formed at a period of about 200 nm to 240 nm and having a depth of about 20 to 50 nm. In addition, the DFB laser diode is formed on a semiconductor substrate. As the semiconductor substrate, a compound semiconductor substrate such as an InP substrate is used. However, a surface of the compound semiconductor substrate is not completely flat and has some undulation (surface roughness) of about 0.1 μm (100 nm). When a pattern formed on the mold is transferred to a resin layer by pressing a mold on the resin layer, variation occurs in a pattern shape due to the surface roughness of the substrate. Specifically, variation occurs in the depth or line width of projections and recesses of the resin layer. When a semiconductor layer is processed using the resin layer as a mask, the pattern shape of the resin layer is directly transferred. Consequently, it is difficult to form a desired pattern in the semiconductor layer because of the variation in the pattern shape. Specifically, in forming a diffraction grating structure, when variation occurs in height of the projections and recesses of the resin layer, relatively thin regions of the projections in the pattern disappear. When a diffraction grating is formed on a substrate having some surface roughness thereof by using the NIL technique, it is difficult to form a desired diffraction grating pattern.
In this case, therefore, the resist pattern including two resin layers disclosed in the above-described document of M. Miller, et. al. can be used. First, a first resin layer is formed on a substrate, and then projections and recesses are formed by the NIL technique. Next, a second resin layer is completely buried in the recesses. In this case, the second resin layer is formed to cover the projections and recesses of the first resin layer. Then, the second resin layer is etched (etch-back step) until the projections of the first resin layer are exposed. As a result, there is no difference in level between the first resin layer and the second resin layer. A flat surface over the first resin layer and the second resin layer is obtained. Next, the first resin layer is selectively etched using the second resin layer as a mask to form projections and recesses (reverse pattern). Since the surface of the first resin layer is flat, the recesses and projections (reverse pattern) having a uniform shape can be formed. Therefore, a resist mask having a uniform shape with little variation regardless of the poor flatness of a substrate can be formed. Therefore, a diffraction grating structure can be preferably formed using the two-layer resist mask.
However, it was found that the use of such a two-layer resist for forming a diffraction grating structure has the following problem. That is, in order to sufficiently secure etching selectivity between the first resin layer and the second resin layer, a non-silicon (Si)-containing resin is used for the first resin layer, and a Si-containing resin is used for the second resin layer. However, it is generally difficult to uniformly etch the Si-containing resin layer in the etch-back step before selective etching. As a result, the projections of the first resin layer are not uniformly exposed, leaving the second resin layer as residues on portions of the surface. Therefore, a desired pattern cannot be obtained even by subsequent selective etching of the first resin layer.